The present invention relates to a method for producing a composite structure having a three-dimensionally curved portion and a cylindrical portion, and to a composite structure produced thereby.
Composite prepregs are light in weight and high in strength, so that they have been widely used as a material for automobiles, ships, aircrafts, etc. Aircrafts using the composite prepreg generally contain a composite structure having a three-dimensionally curved portion and a cylindrical portion. Such a composite structure is usually composed of a honeycomb sandwich panel or a stiffened panel.
The composite structures composed of a honeycomb sandwich panel are produced by the steps of: cutting a honeycomb material into a desired shape to obtain a honeycomb core; laminating the honeycomb core with skins made of a composite prepreg on a forming die in a sandwich form to prepare an assembly; and forming the assembly by heating under a pressure. Thus, the composite structures composed of a honeycomb sandwich panel can be produced by relatively simple processes with reduced production cost. However, such composite structures are often disadvantageous in that sufficient weight-reducing effect is not achieved.
On the other hand, the composite structures composed of a stiffened panel are produced by the steps of: laminating skins and stiffeners (stringers and frames) each made of composite prepregs on a forming die to prepare an assembly; and forming the assembly by heating under a pressure using a pressure bag, etc. The stiffened panels, particularly such that the skins and the stiffeners are integrally formed, are superior in weight-reducing effect to the honeycomb sandwich panels. However, in the case where the stiffened panel is produced by integrally forming the skins and the stiffeners, the stiffeners are preformed and then integrated with the skins while using a jig on the die, thereby increasing production cost. In particular, when the stiffened panels are used for the three-dimensionally curved portions, the production cost is remarkably increased.
An object of the present invention is to provide a method for producing a sufficiently lightweight composite structure having a three-dimensionally curved portion and a cylindrical portion with reduced production cost, and a composite structure produced by the method.
As a result of intensive research in view of the above object, the inventor has found that a sufficiently lightweight composite structure having a three-dimensionally curved portion and a cylindrical portion can be produced with reduced production cost by forming the three-dimensionally curved portion composed of a honeycomb sandwich panel and the cylindrical portion composed of a stiffened panel at the same time. The present invention has been accomplished by the finding.
Thus, a method of the present invention for producing a composite structure having a three-dimensionally curved portion and a cylindrical portion comprises the steps of: (a) placing an outer skin made of a composite prepreg on a forming die; (b) mounting a honeycomb core on a portion of the outer skin that forms the three-dimensionally curved portion; (c) superimposing an inner skin made of a composite prepreg thereon; (d) arranging a preformed frame member and a preformed stringer member each made of a composite prepreg on a portion of the inner skin that forms the cylindrical portion to prepare an assembly; and (e) forming the assembly by heating under a pressure.
The honeycomb sandwich panel is generally formed under a pressure of approximately 3 kg/cm2, and the stiffened panel is generally formed under a pressure of approximately 6 kg/cm2. Conventionally, when the honeycomb sandwich panel and the stiffened panel are formed at the same time under a pressure of 3 kg/cm2, the stiffened panel is deteriorated in its quality. On the other hand, when they are formed at the same time under a pressure of 6 kg/cm2, an edge portion of a honeycomb core contained in the honeycomb sandwich panel is often unable to withstand the pressure, resulting in deformation (crush) thereof.
Therefore, in the present invention, the above assembly is preferably formed by heating under a pressure while disposing an elastic plate and a press plate on a fin portion where the outer skin and the inner skin overlap each other, to prevent the crush of the honeycomb core. Also, a fin portion having a width of 500 mm or more can sufficiently prevent the crush of the honeycomb core without the elastic plate and the press plate. Thus, in this invention, the honeycomb sandwich panel and the stiffened panel can be formed at the same time under a pressure of approximately 6 kg/cm2 without the crush of the honeycomb core and insufficient forming. The pressure is preferably 6 to 7 kg/cm2.
A plurality of the preformed frame members may be linearly arranged at an interval on the portion of the inner skin that forms the cylindrical portion, one or more of the preformed stringer member passing through the interval and intersecting the preformed frame members. In this case, it is preferable that the assembly is formed by heating while using a forming jig comprising a combination of an elastic jig engageable with the intersection of the preformed frame members and the preformed stringer member, and rigid jigs attachable to the members in portions other than the intersection.
The method of the present invention can be suitably used for producing an aircraft body. Further, a composite structure of the present invention is obtainable by the method.